This invention relates to phosphor coatings applied on the inner wall of electrical discharge devices and particularly those containing phosphors such as oxides, silicates, germanates, tungstates and phospho-vanadates which are known to cause the gelation of most water-base coating suspensions.
In the conventional method of applying a film of phosphor on the inner wall of fluorescent lamps the phosphor, or phosphor mixture, is suspended in a viscous medium consisting of an organic binder dissolved in a solvent.
The phosphor suspension which also contains an inorganic binder is flushed down the glass envelope and dried by applying heat and air to evaporate the solvent. The dried coated bulbs are then subject to a lehring process at a temperature of approximately 600.degree. C. in order to burnout all the organic matters and then proceed to the conventional finishing operations of the fluorescent lamp manufacture. Economic considerations and compliance with environmental regulations have emphasized the advantages of water-soluble binders over organic solvent binders in the preparation of phosphor coatings.
The stability of water-base coating suspensions is of primary importance not only to achieve even smooth phosphor films but also to avoid the loss of very expensive phosphors such as used in the high efficiency lamps. Examples of such phosphors are: Europium activated Yttrium oxide, Terbium-Cerium activated Magnesium Aluminate, Europium activated Barium Magnesium Aluminate and mixtures thereof.
Difficulties in maintaining the phosphor coating stability arise when the slight solubility of the phosphors releases polyvalent cations which cross-link with the organic binder and cause the gelation of the coating.
In British Pat. No. 1,467,724 this lack of stability is overcome by the use of hydroxy-propyl cellulose. In British Pat. No. 1,506,334 the use is taught of an ammonium or magnesium alginate water-soluble binder.
Beside stability phosphor coating suspensions have many requirements reflected by the number of patents already in existence.
For instance it is desired to achieve the proper viscosity with the minimum amount of binder. This is usually the case when the binder is carboxy methyl cellulose as in U.S. Pat. No. 2,824,814.
In U.S. Pat. No. 2,987,414 the use of poly vinyl methyl ether and maleic anhydride is claimed to produce a densely packed phosphor film. This organic binder however requires relatively higher lehring temperatures detrimental to the fluorescent lamp efficiency.
Polyoxyethylene is a water-soluble binder of medium high viscosity. Its ash content is well over 1% and contains a high proportion of silica, one of the most detrimental contaminants in fluorescent lamps.
Hydroxyethyl cellulose and many other water-soluble cellulose derivatives tend to produce grainy coatings. Homopolymers and copolymers of acrylic acid are hard to bakeout and tend to leave a slight discoloration of the phosphor film.
Another aspect of the water-base phosphor coatings is the lack of adherence after the lehring operation which causes a high shrinkage during the manufacture and loss of finished products due to peeling off during transportation.
Additions of a permanent inorganic binder are claimed in U.S. Pat. Nos. 3,424,605 and 3,424,606 by using Ba(NO.sub.3).sub.2. The easy conversion of the nitrate into carbonate however tends to void the effect of the addition.
Barium phosphate and phosphoric acid claimed in U.S. Pat. No. 3,551,180 are detrimental to the maintenance of the fluorescent lamps and the same applies to the addition of Sr.sub.2 P.sub.2 O.sub.7 claimed in U.S. Pat. No. 3,636,352.
It is an object of this invention to provide a method for preparing a water-soluble binder having none of the aforesaid drawbacks.
A further object is to provide a method of achieving a stable dispersion of inorganic binder into said water-soluble binder.
Still another object is to provide a method of preparing a universal water-base coating suspension applicable to all phosphors and particularly those which tend to release polyvalent cations under hydrolysis.